1. Field of the Invention
The present invention describes methods for patterning a substrate to create a positional and compositionally well defined nanostructures and nanostructure arrays on a wafer scale. The patterned nanostructures have applications in biological and chemical sensing, catalysis, nanocomputing and nanoelectronics.
2. Related Art
Nanostructures are structures with characteristic dimensions less than 100 nm, i.e. nanoscale. Matters can exhibit size dependent properties at this scale due to physical laws governing microscopic objects, which enable novel applications. Despite their novel property, however, an economic and production worthy method of fabricating nanostructures and nanostructure arrays on substrate and making necessary connections for functional devices has yet to be found.
Methods for fabricating nanostructures on substrate fall into two categories, top-down and bottom-up approaches. Conventional top-down approaches such as photolithography, electron beam lithography or ion beam lithography involve creating structures by forming and removing unwanted parts of various films. They are prohibitively expensive for forming devices less than 100 nm. The resolution of photolithography also suffers from the physical limit of the wavelength of the electromagnetic radiation. Although 30 nm resolution has been demonstrated with extremely short electromagnetic radiation, the cost of such a tool is prohibitively expensive. Electron beam lithography and ion beam lithography boast better resolution down to 10 nm, but these methods require the energetic particle beam to visit each spot sequentially and greatly limit the number of devices that can be made in an industrial production environment.
Other novel lithography methods have emerged over the years. Direct write approaches use scanning probe microscopy to deposit molecules on or change physical or chemical properties of the substrate in the proximity of the probe. These methods suffer from slow speed and lack of a general applicability to fabricate functional devices, which limit their use for industrial production.
Imprint lithography is known in the art whereby a mold with nanostructures is pressed into a thin polymer film on a substrate, which retains the thickness contrast relief pattern after removal of the mold. Further process is then used to transfer the pattern into the whole resist. However, imprint lithography does not generate a pattern; it only transfers an existing pattern on the mold. To generate the nanostructure pattern on the mold, another lithography method is required, preferably electron beam lithography. This limitation makes the mold fabrication very expensive and imprint mold made today has very low surface coverage of nanostructure-patterned area.
In bottom-up approaches, nanoscale building blocks are synthesized initially by chemical methods, then selectively added to (rather than removal from) a substrate, followed by making contacts to make a functional device. However, these approaches have limitations on the degree of control they can achieve on each stage of the device fabrication and are not suitable for industrial production.
Nanoscale building blocks that can be chemically synthesized include carbon annotate, nanowire, nanocrystal, nanorod, etc. Most of the synthesis processes produce a multitude of nanoscale products, which have different dimension, composition and shape, therefore, different physical and chemical properties. For example, heterogeneous nanowires synthesized by chemical vapor deposition are shown in Chem. Mater. 2000, v 12, p 605-607 (Wu, Y. and Yang, P.). To control the diameter, researchers control the diameter of the catalyst used to grow them, which only shifts the center of a broad distribution of the diameters of materials synthesized, as shown in FIG. 2 of Appl. Phys. Lett. 2001, v 78, p 2214. To dope the nanowires, investigators introduce gases containing the dopant during the synthesis, as in J. Phys. Chem. 2000, v 104, p 5213. Although no reproducibility is reported, the uniformity of doping is not expected to be good due to the difference in growth rate related to the broad diameter distribution of nanowires.
Assembly of a nanostructure device by depositing building blocks on a substrate for further processing is currently done by liquid assisted methods, such as Langmuir-Blodgett film or fluidic alignment through micromolded channels. They are described in Nano Letters, 2003, v 3, p 1255, Science, 2001, v 291, p 630, Science, 2001, v 291, p 851. These prior art methods suffer from the fact that they can not achieve the degree of position control the modern semiconductor industry has down to 0.01 μm, as shown in FIGS. 2 and 4 of Science 2001, v 291, p 630, where broken, double, and tilted lines are seen. The positions are not highly controlled. Another drawback for these methods is that it is still experimental and has a difficult time to scale up for production.
The final step in making nanostructures into a functional device usually requires an electrical connection to a signal processing unit. The lack of positional control in the bottom-up approach in depositing the nanostructures on the substrate makes the final connection-making step difficult. Often the position of each nanostructure has to be determined by microscopy methods such as scanning electron microscopy or atomic force microscopy; then electron beam lithography or other serial lithography is used to make the connections. Or alternatively, preexisting contact structures are fabricated with top-down methods; nanoscale building blocks are then deposited onto these structures with contacts established by chance. Although small quantities of samples have been built using these techniques for lab studies, they are not suitable for large-scale manufacturing.
To address this problem, a novel method is invented to produce high-density nanowire arrays, Science, v 300, p 112. Although this method eliminated the nanowire synthesis step and the assembly step, since they have control over the nanowire position, it is still not a production-worthy method on the industrial scale. The method relies on the selective removal of AlGaAs in the AlGaAs/GaAs superlattice, and uses the superlattice as a mold to deposit materials on wafer. The area that can be patterned is defined by the superlattice thickness; to grow a superlattice up to 100 μm is a very time-consuming step. Even so, they can only pattern a 100-μm area at the most, and thus exhibit a slow throughput process. The method is also limited to produce only straight nanowire patterns, and not any other shapes.
Sidewall image transfer (SIT) was invented by the semiconductor industry in early 1980s to produce sublithography images and spaces for the formation of polysilicon gate in sub-micron range. Briefly, a vertical step is created on a planar substrate, which is covered by conformal deposition of a silicon oxide, preferably silicon dioxide or nitride. The resulting gate length is approximately equal to the thickness of the layer deposited. More details can be found in U.S. Pat. Nos. 4,358,340, 4,419,809, 4,419,810, 5,139,904, and 5,795,830. A more recent application of this method is towards the fabrication of FinFET, as disclosed in IEEE Device Letters, 2002, v 23, p 25 and Solid-State Electronics, 2002, v 46, p 1595, the contents of which are hereby incorporated by reference in its entirety for all purposes.
Despite its ability to create sublithography patterns, SIT has only been applied to specific materials, namely Si and polysilicon, and to specific devices such as transistors. The minimum feature dimension can be achieved by this method is on the order of 20 nm and thus limits its application in creating nanostructures.
A need exists for methods that can fabricate positional, compositionally and shape controlled nanostructures and nanostructure arrays on a substrate to enable large-scale manufacturing.
Imprint lithography can transfer nanoscale patterns on a large area, but it can not generate patterns. Existing methods for fabrication of nanostructures with imprint lithography depend on electron beam lithography to make the nanoscale patterns in a mold, which can be very expensive for mold with high nanoscale pattern coverage. The present invention contemplates that nanoscale patterns can be made on a substrate, which can then be used as mold in imprint lithography.
Nanowire biosensors have been discussed in the literature, Science 2001, v 293, p 1289 and Nano Letters, 2004, v 4, p 51, the content so both are hereby incorporated by reference in its entirety. These sensors are assembled from CVD-synthesized nanowires, and they are difficult to manufacture on an industrial scale. Sensors of the prior art do not have the compositional and positional control over the nanostructures that arrays and sensors made in accordance with the present invention possess. In general, the sensors include nanotubes or nanowires in contact with electrodes, thus forming a circuit for current flow. The sensing occurs when analytes contact the nanotubes or nanowires. Chemical sensors based on TiO2 nanowire are also reported. The present invention contemplates that nanoscale sensing devices may be manufactured cheaply in mass.